Actuating an internal combustion engine with an interference signal and evaluating the reaction of the internal combustion engine with the purpose of detecting the operating state of the internal combustion engine is known from the German patent application with the file reference 102008041108.6.
Vehicle drive trains are usually not equipped with sensors which permit direct measurement of currently generated drive torques. Fault-free operation of a drive assembly or satisfactory generation of torque therefore cannot be determined directly. In the case of internal combustion engines, satisfactory combustion, in particular at a low load, can only be detected with difficulty. Available measurement signals for internal combustion engines are usually not sufficient. In the case of unsatisfactory combustion there is a risk of unburnt fuel passing into the oil circuit or the exhaust section and causing damage or a fire, for example during relatively long downhill travel of a conventional vehicle.
In the case of hybrid motor vehicles, the vehicle drive train is composed of an internal combustion engine and at least one further drive assembly, for example an electric machine or a hydraulic engine. During driving with an internal combustion engine which is switched off it may be necessary to start the internal combustion engine, for example if the driver requests more power than the further drive assembly can supply. Since in the case of a hybrid vehicle the internal combustion engine can be entrained by a further drive assembly, the consequences described above are also possible if the satisfactory combustion does not start. In order to avoid these consequences it is necessary to monitor the satisfactory functioning of the components, of the drive assemblies and of the entire vehicle drive train and therefore to monitor the satisfactory generation of torque thereby. Negative effects on the driving comfort and the driving safety can be avoided in this context, even in the case of low assembly loading.